Color filters can be made by a wide variety of techniques. They may be based on the principles of interference or they may be based on absorption of light. In the former case, they comprise thin layers and the transmission spectrum of the filter depends on the thickness of the various layers and may also depend on the viewing angle. In the latter case, the absorptivity is an inherent property of the material from which the filter is made and the color is independent of viewing angle. Glasses are particularly convenient for making color filters because of the ease with which they are fabricated in any arbitrary shape. Furthermore, glasses can be quite robust, exhibiting both chemical durability and reasonably good scratch resistance.
Glass color filters form a special subgroup of colored glasses. In many applications of colored glasses, aesthetics impose the requirements of the color. In the applications of color filters, it is desired that the transmitted light be confined to a special spectral region. For example, it may be desired that no light of wavelength shorter than some particular value be transmitted. A filter that fulfills that requirement is sometimes called a "short wave cutoff filter." A filter transmitting no light of wavelength longer than .lambda..sub.1 or shorter than .lambda..sub.2 is called a "band pass filter."
Absorption of light may be achieved by a variety of phenomena. For example, light might be absorbed by stimulating transitions within a single atom. The color produced by the transition metal ions in glass exemplify this mechanism. The intensity of the color produced by this means is normally quite weak and is confined to a relatively narrow band of wavelengths. Thus, glasses colored by this means are not often useful color filters. Alternatively, very strong absorption can be achieved in glass by the precipitation of colloidal metal particles such as silver. However, as described by R. J. Araujo, and S. D. Stookey, Applied Optics, 7 (1968), pg. 778, the absorption bands are rather narrow and such glasses are not good color filters. According to Araujo and Stookey, in polarizing glasses containing silver colloids, the widths of the absorption bands are of the order of one hundred nanometers. In glasses which have not been stretched to make them polarizing, the absorption would be very similar to the short wave length band.
Crystalline insulators or semiconductors act effectively as short wave cutoff filters because they do not absorb light less energetic (long wavelengths) than their band gap and strongly absorb all light more energetic than their band gap. However crystalline materials are not always easily fabricated in large sizes.
If sufficiently small crystals can be precipitated in glass, absorption at short wavelengths without objectionable scattering at long wavelengths can be achieved. The precipitation of cadmium sulfoselenides in glass has long been used to make color filters. U.S. Pat. No. 5,281,562 provides a more recent example of a short wave cutoff filter which comprises very small crystals suspended in a glass matrix. In the '562 patent, the suspended crystals of cuprous halides strongly absorb all wavelengths shorter than about 400 nm and transmit essentially all of the light at longer wavelengths.
In addition to their usefulness as color filters in the ordinary sense, both of the above examples exhibit another useful property. In each case, the spectral transmittance of the filter is altered somewhat when irradiated with sufficiently intense light. This non-linear optical property is potentially useful for a variety of purposes such as optical switching. There continues to be a need for new color filters having cutoffs appropriate to specific applications. Accordingly, it is the object of the present invention to provide a color filter which exhibits non-linear optical properties.